We solved this problem. We can vary our pulse width from 0% to 40%(approx). But we have another problem.
The PWM output frequency does not match with that calculated. Formula for frequency of generated PWM is f=1/(CT*(0.7*RT+3*RD).
when RT=15k, CT=1nF, RD=22 Ohm. Frequency should've been 95KHz. But output frequency was around 41KHz.
also for RT=4.7k, CT=1nF, RD=470 Ohm.Frequency should've been around 266KHz. But our output frequency was around 155KHz.
We do not understand if we have done amy mistake here.
RT was connected between pin 6 and ground.
CT was connected between pin 5 and ground.
RD was connected between pin 5 and 7, as mentioned in the datasheet.